build-llvm: Set LLVM_HOST_TRIPLE when cross compiling

This makes the bare executables know what their default triple
is.

build-llvm.sh

@@ -141,6 +141,7 @@ cmake \
     -DLLVM_TARGETS_TO_BUILD="ARM;AArch64;X86" \
     -DLLVM_INSTALL_TOOLCHAIN_ONLY=$TOOLCHAIN_ONLY \
     -DLLVM_TOOLCHAIN_TOOLS="llvm-ar;llvm-ranlib;llvm-objdump;llvm-rc;llvm-cvtres;llvm-nm;llvm-strings;llvm-readobj;llvm-dlltool;llvm-pdbutil;llvm-objcopy;llvm-strip;llvm-cov;llvm-profdata;llvm-addr2line" \
+    ${HOST+-DLLVM_HOST_TRIPLE=$HOST} \
     $CMAKEFLAGS \
     ..